The present invention relates to a method of producing a rod-shaped base material for an optical transmission fiber and, more particularly, to improvements in a method of producing a rod-shaped base material for an optical transmission fiber fabricated by a vapor phase axial deposition process (VAD process) in which the refractive index distribution of a light guide core is of a graded index (GI) type.
An optical transmission or telecommunication fiber used for light transmission or telecommunication having a wide band and low transmission loss was heretofore fabricated by the steps of first forming a rod-shaped base material accommodating a similar cross sectional refractive index distribution to that of an optical transmission fiber by, for example, an internal chemical vapor deposition process (an internal CVD process), an external chemical vapor deposition process (an external CVD process) or a vapor phase axial deposition process (a VAD process) and then thermally spinning the rod-shaped base material thus formed by a spinning machine.
Thus, as shown in FIG. 1, the conventional method includes first, the stop of introducing into a reaction zone SiCl.sub.4 gas capable of becoming optical transmission glass soot by an oxidation reaction and a gas such as GeCl.sub.4 gas, POCl.sub.3 gas or BBr.sub.2 gas capable of becoming a dopant for varying the refractive index of the optical transmission glass and contained in the reaction zone. Also oxy-hydrogen flame burners 1 and 2, are used for reacting these gases. The second step consists of sequentially bonding the glass soot 3 consisting of SiO.sub.2 and GeO.sub.2, P.sub.2 O.sub.5 and/or B.sub.2 O.sub.3, which is produced towards the end zone 5 of a quartz starting material 4 and thus fabricating a rod-shaped soot-containing glass unit 6 which later becomes a light guide core. The starting material 4 is rotated at a slow rate of speed around the longitudinal cylindrical axis 0 of the reaction vessel. In this case, the rod-shaped glass unit with soot, i.e., the soot-containing glass unit 6 is moved along the axis 0 at the end where the soot 3 is bonded or built up from the burners 1 and 2 and the soot-containing glass unit 6 has a predetermined constant diameter. The burner 1 is disposed coaxially with the axis 0, and the burner 2 is so disposed as to be slightly inclined with respect to the axis 0. The burner 1 forms the glass soot which has a high refractive index and the burner 2 forms the glass soot which has a low refractive index so that the rod-shaped soot-containing glass unit 6 thus formed has a gradually decreasing refractive index from the axial center toward the outer peripheral surface thereof.
The rod-shaped soot-containing glass unit 6 having the foregoing refractive index distribution can be formed similarly with only one burner 7 as indicated in FIG. 2 or with more than three burners by suitably forming the oxy-hydrogen flame temperature and the burner structure.
The conventional method also includes a third step of heat treating the rod-shaped soot-containing glass unit 6 thus formed so that the refractive index is graudally decreased from the central axis toward the outer peripheral surface thereof in a helium gas atmosphere, thereby forming a transparent glass unit in which the refractive index is gradually decreased from the central axis toward the outer peripheral surface. In this step, an OH group increasing the optical transmission loss in the light wave such as a light having a wavelength of the order of 1.39 .mu.m can be removed by prior heat treatment during the second step of forming the transparent glass unit.
The conventional method further includes a fourth step of coating a quartz glass tube having an inner diameter slightly larger than the outer diameter. By this coating step the refractive index is gradually decreased from the central axis toward the outer peripheral surface. There is also a fifth step of heating and softening the quartz glass tube, reducing its diameter so that a rod-shaped base material for an optical transmission fiber in which a quartz glass is covered on a transparent glass unit is formed. In this case, the transparent glass unit may also be covered with a predetermined quartz glass tube after the transparent glass unit is first thermally oriented and is thus formed in a transparent glass unit having a slender shape with a reduced diameter. The outer diameter of the optical transmission fiber and the diameter of the core can be readily controlled by these steps.
The conventional method may have an alternative fourth step instead of the previously described fourth step, of coating quartz glass soot produced by an oxidation reaction in a predetermined thickness on the transparent glass unit, and an alternative fifth step of heating and sintering the glass soot to thereby form a transparent glass, thereby covering the quartz glass on the outer periphery of the transparent unit. The quartz glass thus covered on the outer periphery of the transparent unit serves to mechanically protect the core of the optical transmission fiber formed by thermally spinning the rod-shaped base material for the optical transmission fiber and to further maintain the transmission characteristics for long time. The rod-shaped material for the optical transmission fiber thus produced is formed in an optical transmission fiber having a predetermined diameter by later thermally spinning the base materials with a spinning machine.
Although the rod-shaped base material for an optical transmission fiber thus formed in a grades index (GI) type with the refractive index distribution of the core by the conventional vapor phase axial deposition (VAD) process has heretofore been produced, it is necessary to form the refractive index N(r) of the core of the optical transmission fiber thus formed by thermally spinning the rod-shaped base materials so that the value of .alpha. in the following formula becomes "2" in order to increase the band of the optical transmission fiber: ##EQU1## where N.sub.0 represents the refractive index of the central part of the core,
N(r) represents the refractive index of the outer peripheral surface of the core, PA1 a represents the radius of the core, PA1 .alpha. represents (No-N(r))/No, and PA1 r represents the distance from the central axis (center).
In order to set the value .alpha. of the refractive index of the core of the optical transmission fiber to "2", when forming the rod-shaped glass unit 6 with soot, it is necessary to proper select, the relative position between the burners 1 and 2, the amounts of gases supplied to the respective burners 1 and 2, the relative distance between the respective burners 1, 2 and the end of the soot-containing glass unit 6 of the side to which the glass soot 3 is sequentially bonded, the temperature of the oxy-hydrogen flames of the respective burners 1 and 2, and the structure of the respective burners 1 and 2. However, it was very difficult to set these all values to proper levels. Since the burners 1, 2 are consumable, it was impossible to set the value of .alpha. over the entire core in radial direction to "2" because, when the burners 1, 2 are replaced, all the values must be reset to revised values. Actually, the value of .alpha. is set to between 18 to 22 for "2", and such rod-shaped glass soot value of .alpha. is set in the vicinity of "2" in the central part but the value of .alpha. is set largely to a value other than "2" in the peripheral part because of the refractive index distribution of the flared shape which produces a multiple number. Thus, it was impossible to produce an optical transmission fiber with a wide band.